Node.js Crypto, what's the default padding for AES?

Question

I've traversed the Node.js Crypto documentation but still couldn't find the default padding used by the Cipher class, for example the method cipher.setAutoPadding(true) has no specification about it. So is it PKCS#5, PKCS#7...?

Any info on this will be great!

Answer 1

In the documentation (https://nodejs.org/api/crypto.html#crypto_cipher_setautopadding_autopadding) it says:

Disabling automatic padding is useful for non-standard padding, for instance using 0x0 instead of PKCS padding.

So it's using "PKCS". More specifically, PKCS7.

PKCS7 defined the same padding algorithm that PKCS5 did, but PKCS5 assumed all ciphers would have 8 byte (64 bit) block sizes. PKCS7's version describes it as a k-byte block. In practice, people ignore that PKCS5 had a fixed block size, and "PKCS5 padding" and "PKCS7 padding" are the same thing.

PKCS5 (https://www.rfc-editor.org/rfc/rfc2898#section-6.1.1):

4. Concatenate M and a padding string PS to form an encoded
     message EM:

             EM = M || PS ,

     where the padding string PS consists of 8-(||M|| mod 8) octets
     each with value 8-(||M|| mod 8). The padding string PS will
     satisfy one of the following statements:

             PS = 01, if ||M|| mod 8 = 7 ;
             PS = 02 02, if ||M|| mod 8 = 6 ;
             ...
             PS = 08 08 08 08 08 08 08 08, if ||M|| mod 8 = 0.

PKCS7 (https://www.rfc-editor.org/rfc/rfc5652#section-6.3):

Some content-encryption algorithms assume the input length is a
multiple of k octets, where k is greater than one.  For such
algorithms, the input shall be padded at the trailing end with
k-(lth mod k) octets all having value k-(lth mod k), where lth is
the length of the input.  In other words, the input is padded at
the trailing end with one of the following strings:

                 01 -- if lth mod k = k-1
              02 02 -- if lth mod k = k-2
                  .
                  .
                  .
        k k ... k k -- if lth mod k = 0